In recent years, there has been an ever increasing degree of fineness of detail in fabrication of semiconductors and fabrication of substrates on which semiconductor chips are mounted, and higher and higher resolutions have been demanded of the projection and other such exposure apparatuses that expose the patterns thereon.
To satisfy this demand, it is necessary to shorten the light source wavelength and increase the NA (the numerical aperture) of the projection optical system. However, as the wavelength is shortened, the number of optical glasses capable of standing up to practical use becomes limited due to absorption of light. At wavelengths of 180 nm and shorter, the only glass permitting practical use is fluorite. Moreover, in the deep ultraviolet and x-ray domains, there may be no optical glass at all that is capable of being used. In such situations, it becomes impossible to construct a reduction projection optical system either from an entirely dioptric optical system or a catadioptric optical system.
For this reason, a number of so-called catoptric reduction optical systems, wherein projection optical systems are constructed entirely from catoptric systems, have been proposed, such as those disclosed in Japanese Laid-Open Patent Application (Kokai) No. S52[1977]-5544, Japanese Patent No. 2603225, U.S. Pat. No. 5,063,586, U.S. Pat. No. 5,153,898, U.S. Pat. No. 5,220,590, U.S. Pat. No. 5,353,322, U.S. Pat. No. 5,410,434, and Japanese Laid-Open Patent Application (Kokai) No. H9[1997]-211332.
Of the aforementioned proposed art, that disclosed in the first, Japanese Laid-Open Patent Application (Kokai) No. S52[1977]-5544, is basically a variation on the Offner catadioptric optical system, a well-known aplanatic optical system of magnification unity consisting of concentric mirrors. This optical system consists of two elements, a first spherical mirror and a second spherical mirror, the respective radii of curvature of which are in the ratio 2:1 and the respective centers of curvature of which are coincident. In addition, light rays leaving an object point in a plane perpendicular to an optical axis passing through the centers of curvature are incident on the first spherical mirror which has a concave surface so as to be parallel to the optical axis, are reflected, are incident on the second spherical mirror which has a convex surface, are again reflected, are incident on a third spherical mirror, and are reflected therefrom to form an image in aplanatic fashion at an opposing point symmetrically located with respect to the optical axis.
While the case described in the aforementioned art is in terms of an image magnification of unity, if the object point is moved from the aforementioned plane perpendicular to the optical axis in a direction away from the mirrors, the imaged point will move from the plane perpendicular to the optical axis in a direction toward the mirrors, creating a reduction projection system. However, because a unity magnification system is being made into a reduction system, the location of the stop must be shifted. In addition, the first spherical mirror which has a concave surface and which was twice used as a reflective surface must be made into separate mirrors as appropriate to accommodate that reflection, and magnification must be adjusted. Naturally, the optical system will no longer be aplanatic, so to compensate for this, reflective surfaces should be made aspheric and use should of course be restricted to a ringfield zone.
While the basic structure is as described above, in the system as described the object and the image will be on the same side. If a configuration wherein object and image are on opposite sides is desired, because there will have to be a even number of mirrors the optical path is sometimes flipped by inserting a single plane mirror at the interior thereof or a convex surface is added at the enlargement side and the number of degrees of freedom thereby increased. The last of the prior art examples, Japanese Laid-Open Patent Application (Kokai) No. H9[1997]-211332, discloses what are basically two of the aforementioned reduction projection optical systems, each with a concave-convex-concave mirror configuration, lain in series, with an intermediate image being formed therebetween.
Because the projection optical system at Japanese Laid-Open Patent Application (Kokai) No. H9[1997]-211332 contains a total of six reflective surfaces, there is a large number of degrees of freedom for correction of aberration. However, there is the problem that the excessive number of reflective surfaces results in large optical losses. Moreover, imaging performance of the overall projection optical system is lowered as a result of the aberrations produced by the manufacturing errors of the reflective surfaces therein. Accordingly, there is the problem that an excessive number of reflective surfaces necessitates that tolerances for each of the reflective surfaces therein be held to extremely tight values, making manufacture difficult. Accordingly, while imaging performance is theoretically high from an optical design standpoint, the imaging performance of the device that is actually manufactured may be inadequate.
As described above, a catoptric optical system having a concave-convex-concave three-element construction forms the basis for these optical systems, and these constructions excel to a corresponding extent. However, when it becomes desirable to further increase optical system performance, the central convex mirror proves disadvantageous. This convex mirror imparts divergence to the light rays from the concave mirrors at either side thereof, and to accommodate those concave mirrors at either side thereof there is a tendency for its power to be made too strong (excessive refractive power magnitude), resulting in a configuration that is liable to produce aberration.